Fibronectin and integrin receptors play important roles in such processes as embryonic development, wound healing, and the progression of cancer. Techniques involving monoclonal antibodies, molecular and cell biology, and physical biochemistry are used to elucidate molecular mechanisms of fibronectin-receptor interactions with the long-term goals of producing novel bioadhesive substrates and developing rational bases for medical intervention in diseases involving abnormal cellular adhesion and migration. The central cell-binding region of fibronectin requires two distinct sequences for activity: an RGD sequence and a synergistic sequence. A 20 kDa fibronectin cell adhesive fragment of human fibronectin and containing both the RGD and synergy cell adhesive sites has been cloned and expressed. Although the fragment is highly active in soluble form, it has only poor activity when adsorbed directly onto plastic substrates. Full cell adhesive activity can be recovered if the 20 kDa fragment is bound to a non-inhibitory anti-fibronectin antibody pre-adsorbed onto plastic, suggesting that proper presentation of small fibronectin fragments may be important for maximal cell adhesive activity. The structure of a similar murine 20 kDa cell adhesive fragment in solution is being determined by NMR spectroscopic techniques. This is one of the largest polypeptide high-resolution structures attempted to date. Information obtained so far indicates that the synergy and RGD sites do not interact. Certain monoclonal antibodies that bind to integrins can up-regulate their ligand-binding activity. One such activating antibody designated 12G10, appears to bind to a "ligand induced" conformation. The role of the human alpha5beta1 integrin in experimental metastasis has been analyzed using inhibitory anti-alpha5 and anti-beta1 monoclonal antibodies. Both antibodies inhibit metastasis of human breast carcinoma cells in athymic nude mice. The alpha5beta1 integrin may be functioning in several steps of the metastatic cascade including in tumor cell attachment, migration, and extravasation. The modulation of the function of the alpha2beta1 integrin by a panel of human breast carcinoma cells has been examined. This integrin was found to be present and to function in cell adhesion to collagen on all of the cells tested. The non-malignant and/or well differentiated cells also used the alpha2beta1 integrin for adhesion to laminin. In contrast, highly invasive and/or poorly differentiated cells could not, suggesting that the ligand specificity of the alpha2beta1 integrin could be regulated during malignant progression.